Forced Expiratory Volume in One Second (FEV1) DEFINITION:
The volume of air exhaled during the first second of a forced expiratory maneuver. The FEV1 is useful for detecting obstructive diseases since a person with obstructed airways will not be able to exhale as much air in the first second as a person with normal lungs. The FEV1 may also be low if the person has severe restrictive disease.
HOW TO CALCULATE:
1. Find the starting point of a forced expiratory maneuver along the baseline of an acceptable spirogram. A more accurate way for determining the start of the test will be described later in this unit in Section E. Back Extrapolation. This is the point where time (t) = zero. Note the scale on your graph paper to determine the length of one second. Measure the appropriate distance from t = 0 to t = 1 second.
2. Draw a straight line vertically up from the point on the baseline where t = 1 to where it intersects with the curve. Determine the volume on the graph paper at the point of the intersection. This is the FEV1.